Metal-plate lens microwave antenna



April 28, 1959 a s. J. MASON 2,884,629

METAL-PLATE LENS MICROWAVE ANTENNA Filed Nov. 29; 1945 w PLANE IQNVENTOR SAMUEL J. MASON ATTORNEY niwd ews Pa m 2,884,629 lVIETAL-PLATE LENS MICROWAVE ANTENNA Samuel 1. Mason, Boston, Mass, asslgnor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy The present invention relates in general to radio antennas, and more particularly to a directive radio antenna formed by an open ended wave guiding system for producing a collimated beam of radio energy.

As is well known, an open ended wave guide may be used as a radio antenna. However, the energy emerging from such a wave guide usually has a curved wave front, the curvature being most often circular or spherical. A wave front having such a curvature produces a diverging beam which is not suitable for use in directive radio antennas unless modified to become substantially a plane wave front. It is usual to use a reflector or lens of a particular kind to produce a plane or linear wave front that results in a narrow beam of energy.

The present invention is directed specifically to a novel form of electromagnetic wave guide lens for producing electric waves having a linear wave front. Accordingly, a lens structure of novel form is provided in which energy that is introduced therein with a circular wave front has its wave front gradually changed therein and emerges therefrom having a linear wave front, producing a beam which is collimated and narrow in the plane of the lens. The specific novelty of the invention consists of gradually varying the spacing between semi-circular conductive plates comprising the wide walls ofthe lens structure so as to gradually alter the phase velocity of the waves in the lens in a predetermined manner at all regions therein and thereby produce a linear wave front at the radiating opening thereof.

It is accordingly a specific object of this invention to provide an electromagnetic wave guiding lens which gradually changes the phase velocity of waves therein in a predetermined fashion so as to produce a radiant beam of collimated energy.

It is another object of the invention to provide such an electromagnetic wave guiding lens that will produce gradual rather than step-by-step changes in the wave front of the energy passing therethrough, and therefore, will avoid undesired reflections or standing waves.

It is a still further object of the invention to. produce such an electromagnetic wave guiding lens that may be cast or otherwise formed of parts and members that require no external or internal support.

Other and further objects and features of the present invention will become apparent upon the careful conr sideration of the following detailed description when taken together with the accompanying drawing, the figures of which illustrate a typical embodiment of the invention ,and some mathematical graphs which aid in the understanding thereof}, I

In the drawing:

Fig. 1 illustrates in perspective an antenna in accordance with my invention;

Fig. 2 is a side elevation of the apparatus of Fig. 1; Fig. 3 is a front elevation showing the radiating opening of the apparatus of Fig. 1;

Fig. 4 is a cross section taken along the line IV-IV "ofFig.-1;' 5

2,884,629 Patented Apr. 28, 1959 IC C Fig. 5 is a graph illustrating the field configuration and path of propogation of the energy in the lens of Fig. 1;

Fig. 6 is a graph illustrating a conformal transformation used to explain and understand the action depicted in the graph of Fig. 5.

In Figs. 1 through 4 inclusive, a lens structure 10 is formed partially by upper and lower semi-circular walls 11 and 12, respectively. These walls 11 and 12 have circular and diametric edges 13 and 13 and 14 and 14 re spectively. The upper and lower semi-circular walls 11 and 12 are positioned in spaced relationship with their corresponding edges coadjacent, so that the circular edges 13 and 13 have such adjacency while the diametric edges 14 and 14' are likewise coadjacent. The circular edges 13 and 13' are closed by a continuous wall 15 which has an opening 16 in the region of its center. A rectangular wave guide17 feeds energy into the lens 10 through the aforementioned opening 16. The E vectors 18 of energy emerging from the rectangular wave guide 17 are directed in such fashion as to lie in the plane of the lens 10. Thus the rectangular wave guide 17 has its narrow sides 19 substantially parallel to the aforementioned plane. The E vectors 18 are substantially circular upon emerging from the wave guide 17. The longitudinal axis XX of the lens 10 may be considered as lying substantially parallel with a radius of either semi-circular wall 11 or 12, perpendicular to the diarnetric edge 14 or 14'. The

rectangular wave guide 17 has its longitudinal axis coincident with this longitudinal axis XX. Along this axis XX the spacing S between the two semi-circular walls 11 and 12 is arbitrarily chosen to be constant and equal to the distance between the narrow walls 19 of the rectangular wave guide 17. However, in directions perpendicular to the axis XX in the plane of the lens 10, the spacing S between the walls 11 and 12 is gradually diminished with increasing distance away from the axis XX. Thus, toward the outer edges of the pillbox 10, the spacing between the walls 11 and 12 becomes gradually less until the spacing S is had at the extreme edges of the lens at the ends of the opening between the diametric edges 14 and 14'. Since the phase velocity of waves proceeding in or being propagated by a wave guide will be increased as the spacing between the narrow walls thereof is decreased, the phase velocity of waves in the lens will be increased in the region of the outer edges thereof nearest the narrow wall 15. Thus the wave front 18 takes on a gradually increasing radius of curvature until at the opening between the diametric edges 14 and 14 a linear wave front 21 is had, linearity being obtained by properly adjusting the spacing S between the walls 11 and 12 at'all points in the lens 10..

If it is desired to support the semi-circular walls 11 and 12 with internally disposed posts 22, these posts may be placed an odd number of quarter wave lengths apart along a path line 23 of propagation of the energy in the lens 10. The distance L is accordingly preferably an odd number of quarter wave lengths of energy along such a path of propagation. However, the walls 11 and 12 may be cast integral with the narrow wall 15 so that no such supporting posts 22 need be used or if desired external supporting struts of common and ordinary type may be used. Further, it is not necessary that the post 22 be placed as'hereinabove described, but it is believed that such spacing may be instrumental in eliminating or greatly reducing standing waves in the lens 10. Random post positioning is probably equally satisfactory if many small posts are used.

The field pattern of the radiation in the lens 10 may be considered as lying in a plane, the Z plane, bounded by the coordinates OY and OX, where OX corresponds to the axis XX. Fig. 5 illustrates one-half the field pat- 'tern'in the lens 10, but "since the pattern is similar on both sides of the axis X-X, such an illustration is equivalent to a full illustration of the aforementioned field pattern. Thus, as shown in the graph of Fig. 5, the wave front 18 gradually increased in radius of curvature until at the right hand edge, where the value of X equals one, a linear wave front 21 is had. The path lines 23, being the direction of propagation of energy in the regions represented by the respective lines are everywhere perpendicular to the intersecting wave front lines 18 and 21. The configuration represented by Fig. is similar to a section of the field pattern of a pair of bi-polar coordinates.

Any point Z in the Z plane is defined by the relation Z=X+iY where:

X is the magnitude of the coordinate OX, Y is the magnitude of the coordinate OY, and

j is the mathematical operator /1.

Since everywhere in the Z plane path lines 23 are perpendicular to wave front lines 18 and 21 at points of intersection, the system of Fig. 5 is an orthogonal system. This may be transformed by the mathematics of Conformal Transformations into another orthogonal system in simpler polar coordinates in another plane, the W- plane, as shown in Fig. 6, for ease in calculating the spacing at all points between the wide walls 11 and 12. In the W plane a magnitude vector r and an angle 0 (measured with respect to an origin 0 and a reference line OP respectively) serve to define the position of any point W therein, in a conventional manner. Thus any point W in the W plane is defined by the relation:

T67 0 where e is the base of natural logarithms, and j is the mathematical operator mentioned above.

Each point Z in the Z plane may be related to a corresponding point W in the W plane by the arbitrarily chosen transformation relation This relation will provide information from which the spacing S between the upper and lower semi-circular walls 11 and 12 may be so arranged that the straight line wave front 21 is had in the region where X=1 in Fig. 5.

As a result of the above relation (1) points Z in the Z plane become functions of r and 0, so that and the quantities X and Y become also functions of r and B, viz:

Elsewhere the wavelength of the energy in the lens at any point Z in the Z plane is a function of r and 0, and may be expressed as In Order that the linear wave front 21 may be pro- From this Relation 2 it is possible to ascertain the values of spacing S to be assigned at various points 2 in the Z plane in order that at each such point the desirec' ratio lt, iwi!) may exist. Thus:

az e 2W 3( 21's" T' br D r 1-1-16 Whence:

Zo-m 2 or 1+2r cos 6+r Similarly:

DZMO) 2 I a; "1+2'r-l 1' So that:

Mm) -l- (3) M 1+2r cos 6+1" As will be remembered, a point Z in the Z plane is defined by the magnitude of X and Y for that point, in accordance with the relation Z=X+jY and that X and Y are each related to r and 0 in the W plane for the corresponding point W so that X=X and Y=Y The relationships determining these functions are found as follows: Multiply the relation 21a" 1+re" By uni y in the form:

1+rel+reyielding 21's" 1+re'" 2r(e"+ 1') 1+re 1+re l t-re+re-""'-l-r whence:

2r('r+eos 6 I 21' sin 8 1-1-21 cos 0+r 1+2r cos 8+r so that:

2r(r+eos 0) "-""1+2r cos a+r (4) and Y 2r sin 0 1+2? cos 6+1 To find the ratio that exists for each point Z in the 2 plane, it is necessary first to pick the point Z in the Z plane to be considered. This will determine the values of X and Y. Substituting these values of X and Y in the Relations 4 and 5 above,

the resulting two equations maybe solved simultaneously for the corresponding values of r and that determine the corresponding point W in the W plane. These values of r and 6 may then be introduced into the pertinent Relation 3, and the ratio Mil Mao) found by well known arithmetic process. I

As is known by those skilled in the art, the wavelength a in a rectangular wave guide is determined by the relation A 2 ll (a) where: 1

X is the free space wave length of the energy being propagated, and

a is the spacing between the narrow walls of the rectangular wave guide. As will be recognized, this spacing a is the spacing S between the semi-circular walls 11 and 12 of the lens 10, and A is the same as N or in a special case M S is the required spacing at the point Z in question. The value of the ratio V (nfl) Mao) found from the'Relation 3 as set forth hereinabove may then be substituted in this last Relation 6, and the relation solved for the value of the unknown spacing S It will be remembered that the spacing S along the OX axis is constant and predetermined.

As will be appreciated by those skilled in the art, the

transformation equation proposed and used in the above analysis is only one of many useable transformations. For example, the transformation Z= log cot W is suggested. Further, the mathematical method of finding proper spacings between the semi-circular walls 11 and 12 may be circumvented entirely by resorting to empirical processes and thereby obtaining the same result of a linear wave front 21. It will be still further appreciated that the walls 11 and 12 may be made in other shapes than semi-circular, if desired, and that other feeding means than the rectangular wave guide 17 may be used. Accordingly this invention is not to be limited except as is required by the prior art and the spirit of the appended claims.

What is claimed is:

l. A radio antenna comprising, first and second semicircular conductive sheets each having a diametric and a semi-circular edge, said sheets being similarly disposed in spaced relation with similar edges coadjacent, the spacing between said sheets being substantially unvarying along a radius perpendicular to said diametric edges, said spacing gradually decreasing with increasing distance laterally from said radius in directions perpendicular to the direction of said radius, conductive means for electrically connecting together said sheets at said semi-circular edges, and a microwave feed line positioned between the plates at the end of said radius furthest removed from said diametric edges forin troducing into the where r is the free space wave length of the energy being propagated, A is the wave length of said energy in said region along said radius. 7& is the wave length of said energy'desired to be had at said point, S is the spacing between saidsheets along said radius and S is the spacing between said sheets at said point.

3'. A metal plate lens type microwave antenna for radiating an electromagentic wave in a beam having narrow width along a linear wave front comprising a pair of conductive plates of semicircular shape having parallel axes of symmetry, means supporting said plates in gradually diminishing spacing laterally from said axes of symmetry, a relatively narrow wall electrically connecting the outer circumferentialedges of said semicircular plates, and a wave guide feed for introducing electromagnetic microwave energy into the space between said plates in said wall at the circumferential point of maximum spacing.

4. A metal plate lens type microwave antenna for radiating an electromagnetic wave in'a beam having narrow width along a linear wave front comprising a pair of conductive plates of a semicircular shape having parallel axes of symmetry, a conductive wall connecting the outer circumferential edges of said plates, means supporting said plates in a spacing such that the spacing between said plates at any desired point in the region contained therebetween is defined by the relation My A 2 where A is the free space wave length of the energy being propagated, M is the wave length of said energy at the point of maximum spacing, A is the wave length desired at said desired point, S is the spacing along said axis at the point of maximum spacing, and S is the spacing at said desired point.

5. A metal plate lens type microwave antenna for radiating an electromagnetic wave in a beam having n arrow width along a linear wave front comprising a pair of conductive plates of semicircular shape having parallel axes of symmetry, a conductive wall connecting the outer circumferential edges of said plates, means supporting said plates in a spacing such that the spacing between said plates at any desired point in the region contained therebetween is defined by the relation where k is the free space wave length of the energy being propagated, A is the wave length of said energy at the point of maximum spacing, A is the wave length desired at said desired point, S is the spacing along said axis at the point of maximum spacing, S is the spacing at said desired point, and a wave guide feed for introducing electromagnetic energy into the space between said plates in said wall at the circumferential point of maximum spacing.

6- A me al plate l ns type microwave antenna for radiating an electromagnetic wave in a beam having narrow width along a linear wave front comprising a pair of conductive plates each having a semicircular shape about an axis of symmetry, a conductive wall connecting the outer circumferential edges of said plates, said axes being maintained parallel, means in said wall for introducing electromagnetic microwave energy into the region between said plates for propagation along said axes of symmetry, and means to space the region between said plates in gradually diminishing relationship outwardly and perpendicularly from said axes, whereby the energy propagated along longer paths more remote from said axis travels at increasing phase velocity so that all energy emerges from said lens in a linear wave front.

7. A metal lens antenna for converting a circularly cylindrical wave front from a primary radiator to a plane wave front comprising, a pair of spaced metallic plates forming the broad walls of said lens, means for introducing electromagnetic wave energy into the region between said plates such that the plates lie generally parallel to the electrical field thereof, and means for supporting s aid plates with a spacing progressively decreasing from the center of the emerging wave front towards its edges, whereby the phase velocity of energy at the edges of the wave front is greater than at the center.

8. Apparatus for gradually changing the reduction of curvature of thewave front of an electromagnetic wave comprising, a pair of metal plates similarly disposed in spaced relation with their outer edges coadjacent, a relative narrow side wall electrically connecting the outer edges of said plates, means attached to said narrow side wall for introducing electromagnetic wave energy into the region of said plates such that the plates Iie generally parallel to the electric field thereof, and means for supporting said plates with a spacing progressively decreasing from the center of the emerging Wave front toward its edges, whereby the phase velocity of wave energy at the edges of the wave front is greater than at the center.

9. A directive radio antenna for producing a linear wave front comprising, a rectangular wave guide, a pair of arcuate conducting plates forming extensions of the narrow walls of said wave guide, said plates having gradually decreasing separation in directions perpendicular to the line of said extension, and relatively narrow side walls electrically connecting the arcuate edges of said plates.

10. An electromagnetic wave guide lens for producing electrical waves having linear wave front comprising, a rectangular wave guide for the propagation of electromagnetic energy having an electrical field parallel to the narrow walls thereof, a pair of arcuate plates forming extensions of said narrow Walls, said plates having gradually decreasing separation in directions perpendicular to the extension of said wave guide, and relatively narrow side walls electrically connecting the arcuate edges of said plates, whereby the phase velocity of wave energy at the edges of said plates is greater than at the line of extension of said wave guide.

11. Radio antenna comprising a pair of conductive sheets spaced apart to define between them a wave propagating region, the spacing between said sheets being nonuniform, the variation in spacing extending two-dimensionally from point to point according to a law related to a given law of variation in the refractive index of the wave propagating region such that said region contains at least one focal point and radiations having their E vector parallel to said conductive sheets launched into said region at the said focal point will emerge from said region in a desired pattern of radiation.

12. A radio antenna comprising a pair of semi-circular conductive surfaces arranged face-toface in spaced relationship, a semi-cylindrical conductive wall connecting together the semicircular edges of said surfaces, and a feed element adapted to launch radiations having their E vector substantially parallel to the said conductive surfaces, said feed element being associated with said semicylindrical wall substantially at its center, the spacing between the said semicircular surfaces varying two-dimensionally from point to point according to a predetermined law such that the region enclosed between the surfaces contains a focal point at the center of the said semicylindrical wall, whereby radiations entering the said region from said feed element emerge from the opposite boundary of said region in a predetermined pattern.

References Cited in the file of this patent UNITED STATES PATENTS 2,206,683 Wolff July 2, 1940 2,283,935 King May 26, 1942 2,398,095 Katzin Apr. 9, 1946 2,540,839 Southworth Feb. 6, 1951 2,720,588 Jones Oct. 11, 1955 

